This invention relates to composite fusible elements for electric fuses. In this context term "composite fusible element" refers to an element which is fabricated of several parts, preferably parts of silver and copper. The closest prior art known is listed below:
(1) U.S. Pat. No. 2,781,343; 02/12/57 to K. W. Swain for Current-Limiting Fuse Comprising Fuse Links of Silver and Copper.
(2) U.S. Pat. No. 2,809,257; 10/08/57 to K. W. Swain for Composite Fuse Links of Silver and Copper.
(3) U.S. Pat. No. 3,543,209; 11/24/70 to F. J. Kozacka for Composite Fuse Link and Fuse with Composite Fuse Link; and
(4) U.S. Pat. No. 3,543,210; 11/24/70 to F. J. Kozacka for Current-Limiting Fuse Having Fuse Link with Longitudinal Groove.
The object of the above patents is to conserve silver, and to substitute copper for silver in portions of the fusible element where melting i.sup.2 .multidot.t values and vaporization i.sup.2 .multidot.t values are of relatively minor importance. This is also an object of the present invention but not its primary or sole object as will be apparent from what follows.
The manufacture of composite fusible elements involves punching, stamping or blanking operations. However, punching, stamping or blanking cannot normally be effected with a great amount of precision and small dimensional tolerances. As a rule of thumb, when using methods which are conventional in the art of manufacturing fuses, the width of the narrowest point of a fusible element may not exceed the thickness of the fusible element. There has been, and there is, a need to provide fusible elements with points of reduced cross-section that are smaller than the points of reduced cross-section that can readily be formed by a punching, stamping or blanking operation. Punching, stamping or blanking operations tend also to produce stresses in the particular fusible element which are most undesirable. The tendency to produce stresses is particularly large when the geometry of the fusible element is such that more or less sharp angles are formed in it. However, in many instances it is not possible to design fusible elements in such a way that sharp angles are avoided.
Ductile wires substantially circular in cross-section can be drawn so as to have smaller cross-sections than pieces of sheet metal can be reduced in cross-section by conventional punching, stamping or blanking operations. Thus the difficulties involved in the latter process, in particular the setting-up of stresses in a sheet of metal, can be overcome by the use of circular ductile wires.
It is, therefore, the prime object of this invention to provide fusible elements for electric fuses whose points of reduced cross-sectional area are formed by wires, and whose other current-carrying points of large cross-sectional area are formed by pieces of sheet metal.
Another object of this invention is to make the parts of the fusible element which should have small i.sup.2 .multidot.t values of wires, and the other current-carrying parts of the fusible element where the i.sup.2 .multidot.t values are less critical of a sheet metal having i.sup.2 .multidot.t values which may be larger in comparison to those of the wires. Metals which comply with this condition are silver and copper, respectively.
In this context the term copper is intended to include alloys of copper which have substantially the same physical properties as electrolytic copper, and the term silver to include what is considered in the trade as pure silver, though it may have a certain amount of impurities.